In pressurized-water nuclear reactors, the lower core plate supporting the fuel assemblies has, in line with each of the assemblies, water passage orifices passing through it over its entire thickness and an instrumentation guide bush projecting relative to the face of the lower core plate, on which the assemblies come to rest. This guide bush is arranged in such a way that its axis coincides with the axis of the instrumentation tube of the corresponding assembly. The instrumentation guide tube is located in the central part of the assembly and receives the glove finger, within which a teleflex cable carrying a neutron-flux sensor at its end is moved along, in order to carry out flux measurements over the entire height of the assembly while the reactor is in operation. Such measurements can thus be made over the entire height of the core in the region of each of the assemblies
The guide bush projecting relative to the lower core plate forms the end of a guide duct, in which the glove finger can be moved along in either direction to introduce it into the fuel assembly or, conversely, to extract it, for example at the time when the core is being reloaded.
Each guide bush is located, relative to the corresponding assembly, inside the lower end piece, via which the assembly rests on the lower core plate. The guide bush is completely free inside the lower end piece of the assembly, its axis being aligned with that of the assembly solely as a result of the relative positioning of the bush and the assembly on the lower core plate. In pressurized-water nuclear reactors operating at the present time, the guide bushes of the glove fingers, which are fastened to the lower core plate, consequently do not have an accurately defined outside diameter, since this dimension is not critical to ensure that the guide duct is aligned with the guide tube of the assembly.
To make it easier to install the fuel assemblies and allow differential expansion, the upper end of the guide bush is located at a relatively long distance from the entrance of the guide tube of the assembly, within the lower end piece. Consequently, the glove finger, when introduced into the assembly, is exposed, in the free space existing between the guide bush and the end piece, to cross-currents of cooling water which can make it vibrate.
To overcome this disadvantage, it has been proposed, for example, to provide a matching guide unit which caps the guide bush and which bears on the part of the end piece into which the guide tube of the assembly opens.
The fitting of such a matching guide and protective device presents certain difficulties which arise because the guide bushes in the nuclear reactors operating at the present time do not have a perfectly defined and absolutely constant outside diameter. On the other hand, it is difficult to carry out accurate measurements on highly irradiated equipment which has to be placed under a head of water exceeding ten meters during reactor shutdown and maintenance phases.
Finally, the large number of guide bushes arranged on a lower core plate of a nuclear reactor, corresponding to a number of fuel assemblies in excess of one hundred, makes it necessary to use a measuring process and device which can be installed and put into operation quickly and reliably at a distance.
A device making it possible to carry out such remote measurements of outside diameters repetitively has not been known to date.
However, there are various known devices which are used for conducting tests or dimensional checks on nuclear reactor elements, these devices comprising a pole of great length which can be manipulated from a location above the reactor pool; however, such devices have never been designed to carry out accurate diameter measurements.